Establishing how features of the visual world are represented in the activity of cortical circuits, and how these representations are constructed during development remain fundamental challenges for visual neuroscience and are central to understanding the neural basis of visual perception and disorders of cortical function. The experiments in this proposal continue our analysis of the functional organization and development of columnar representations in layer 2/3 of ferret visual cortex. Preliminary evidence suggests an important new dimension to the representation of visual stimuli by layer 2/3 cortical circuits: a columnar architecture composed of neurons that respond preferentially to diffuse luminance increments (ON) or luminance decrements (OFF). By using a combination of techniques for in vivo imaging of functional ON and OFF inputs at the columnar and cellular level of resolution, we will test the hypothesis that ON and OFF responses to diffuse illumination are arranged in a modular fashion in layer 2/3 of visual cortex, explore the cellular basis for thi columnar architecture and its relation to the representation of orientation preference, and probe the contribution of experience independent and dependent mechanisms to its development. Taken together these experiments will yield novel insights into the functional organization and development of the cortical circuits that mediate visual perception, providing the foundational knowledge for addressing a broad range of neurological and psychiatric disorders that impact cortical circuit function.